It is well known that the rate of flotation and overall recovery of copper sulfides, such as those in porphyry ores, are substantially lower with ores which come from upper zones of ore bodies. This decrease in copper recovery and flotation rate is believed due to the weathering effects near the ground surface which results in partial oxidation of the sulfide minerals thereby forming a mixed sulfide and oxide ore. Indeed, severe oxidation can result in the formation in the ore body of zones consisting of copper oxides, carbonates, silicates and hydroxides.
A number of techniques have been proposed for treating copper sulfide ores containing copper oxides, carbonates and hydroxides so as to enhance the total recovery of the metal values. For example, in U.S. Pat. No. 3,883,421, a process is disclosed in which sulfidizing chemicals are added to an ore slurry in water to provide an optimum oxidation-reduction potential in the ore slurry during normal flotation. For copper ores, for example, such optimum oxidation-reduction potential will be in the range of about 125 to 160 millivolts. Similarly, in U.S. Pat. No. 4,011,072, the recovery of oxide and silicate copper minerals with the sulfide minerals is disclosed as being increased by adding a soluble sulfidizing agent to a pulp of the ore which addition is controlled in accordance with the pulp EMF. Specifically, the addition of the sulfidizing agent is disclosed as being discontinued whenever the EMF becomes less than about -30 millivolts as measured against a silver, silver chloride standard electrode.
It should be readily appreciated that in addition to the increase in the total amount of copper that can be recovered from sulfide ores, the rate of recovery or flotation kinetics is also of considerable importance. For example, in commercial practice an improved recovery of upper zone porphyry ores frequently is obtained by increasing the total flotation time so that the slow floating minerals will ultimately float. This practice unfortunately results in higher capital cost. If the additional flotation time, however, is not allowed, then total copper recovery is significantly decreased.
Thus, there remains a need for a process which will enhance the flotation kinetics for slow floating, surface oxidized sulfide minerals, such as the upper zone porphyry ores and which will also increase the overall copper recovery.